1. Field of the Invention
The present invention relates to a cylindrical magnetic recording medium called a patterned medium with a constant track length, a constant recording bit number in each track and high recording density, and to a method for manufacturing the cylindrical magnetic recording medium.
2. Related Background Art
As indicated by high density/large capacity trend in hard disks in recent years, the recording density of magnetic recording media continues to increase at an astonishing annual rate of 60%-100%, and a case has even been reported in which a recording density of over 50 GB/in.2 was achieved. However, further progress is anticipated, and while numerous vigorous research and development efforts continue in this area, an interest in the next generation magnetic recording medium is rising.
At present, the longitudinal recording method in which magnetization is recorded in the direction that follows along a recording surface is used as a magnetic recording method on hard disks. In the longitudinal recording method, leakage field from magnetization transfer regions provided between adjacent magnetized recording parts is used to write and read with a magnetic head. However, in the longitudinal recording method, when bit length is shortened to improve recording density, the leakage field from the magnetization transfer regions becomes smaller, which causes the leakage field to be undetected when the bit length is minuscule. Although this problem can be avoided by making the film thickness of a magnetic layer thinner, this in turn causes bit volume to be extremely small; this situation leads to a superparamagnetic state, in which magnetization direction changes with thermal energy, which makes recorded magnetization impossible to be retained. For these reasons, a recording density of approximately 100 GB/in.2 is considered at present to be the limit using the longitudinal recording method.
On the other hand, the perpendicular recording method, in which a magnetic layer with magnetic anisotropy in the direction that intersects with a recording surface is the recording layer and magnetization is recorded in the direction that intersects with the recording surface, is characterized by diminishing demagnetizing field as the recording density increases, in contrast to the longitudinal recording method. Furthermore, due to the fact that the film thickness of a magnetic layer can be made thicker even when the recording density is increased, the magnetic layer does not fall into a superparamagnetic state caused by thermal energy. For these reasons, the perpendicular recording method is considered to have greater potential than the longitudinal recording method in the recording density region in excess of 100 GB/in.2. Among modes of media that use the perpendicular magnetic recording method are not only disk-shaped magnetic recording media, but also cylindrical, as well as tubular-type media proposed by a research group at Tohoku University. In the perpendicular recording method, a layer of Coxe2x80x94Cr alloy is generally used as the recording layer. When the Coxe2x80x94Cr alloy forms a film through sputtering method on a substrate such as a Si substrate, glass substrate or carbon substrate, the Co and Cr grow in a composition-separation state. Of this, the parts with Co rich composition are columnar and have a hexagonal close-packed structure (an hcp structure), which becomes the parts that retain recording. The parts with Cr rich composition that grow to surround the columnar recording parts are nonmagnetic, and therefore function to weaken magnetic interaction between adjacent recording parts.
In addition, a magnetic recording medium has been devised that has a structure in which a magnetic material, which becomes the columnar recording parts, is embedded in a nonmagnetic material in an artificially regular manner through a micro-fabrication technique. For example, there are magnetic recording media that are produced by having pores formed regularly aligned on a substrate through a series of processing including coating a resist on a glass carbon substrate, patterning with electron beam lithography, and an etching processing; a magnetic material NiFe is then embedded in the pores through sputtering; and the surface is polished so that the magnetic material and the nonmagnetic material form a flat surface. In a magnetic recording medium called a patterned medium that is characterized by having one bit recorded in each of the pores in which the magnetic substance is embedded, due to the fact that the recording parts are the same shape, the same size and regularly aligned unlike the magnetic recording media in the longitudinal recording method or the perpendicular recording method, bit boundaries are regular and the medium has a structure better suited for high density recording. By making the size of the pores and the interval among the pores minuscule, it is possible to achieve a recording density of 1 Tb/in.2, and this recording medium has been gaining attention as the next generation magnetic recording medium.
In order to use a patterned medium as a disk-shaped magnetic recording medium such as hard disks in which a substrate rotates at high-speed, pores embedded with a magnetic substance must be aligned in concentric tracks on the disk-shaped substrate. In a method of concentrically arranging on a disk-shaped substrate pores embedded with a magnetic substance, starting points of anodization are formed by pressing against an Al substrate a male mold (a stamper) with protrusions patterned for pores aligned at a constant interval to form a plurality of concentric tracks having a single center, and the Al substrate is anodized to form pores arranged concentrically.
Further, in a process to form anodized Alumina nanoholes through anodization of Al, the Al substrate is anodized in an acidic electrolytic solution with sulfuric acid, oxalic acid or phosphoric acid to form a porous-type anodized film (see R. C. Furneaux, W. R. Rigby and A. P. Davidson, NATURE vol. 337, p. 147 (1989)). The characteristic of the porous film is that it has a specific geometric structure in which the extremely fine columnar pores (Alumina nanoholes) with diameter of a few nm to a few hundred nm are arranged in parallel at an interval of the few dozen nm to a few hundred nm. The columnar pores have a high aspect ratio and superior uniformity in their cross-sectional diameter. The aspect ratio represents a ratio of the depth y of a pore to the diameter x of the pore, or y/x.
In addition, the structure of a porous film can be controlled to some extent by changing the conditions of anodization. It is known that pore interval can be controlled by anodizing voltage, pore depth by anodizing time, and pore diameter by pore-widening processing, each to some extent. The pore-widening processing is an etching processing of Alumina, which is generally a wet etching processing using phosphoric acid.
Furthermore, a method has been proposed to perform anodization in two stages in order to improve the perpendicularity, linearity and independence of pores of a porous film. In this method, a porous film formed by anodization is removed and another anodization is performed to create a porous film with pores having better perpendicularity, linearity and independence (Japanese Journal of Applied Physics, vol. 35, part 2, No. 1B, pp. L126-L129, 15 Jan. 1996). This method takes advantage of the fact that depressions in an Al substrate, which are formed when the anodized film formed through the first anodization is removed, become starting points to form pores in the second anodization.
Moreover, a method has also been proposed to form starting points to form pores by using a stamper in order to improve the controllability of the shape, interval and pattern of pores of a porous film, i.e., a method for creating a porous film with pores having better controllability of the shape, interval and pattern by using depressions, which are formed by pressing on the surface of an Al substrate a substrate with a plurality of protrusions on the surface, as starting points to form pores in anodization. There is also a method to form depressions that become starting points to form pores by irradiating FIB (focused ion beam) on the surface of a substrate, as well as a method to form depressions that become starting points to form patterned pores by uniformly coating a resist resin on the surface of a substrate, patterning with photolithography or electron beam lithography technique, and performing a dry etching processing.
In order to use a patterned medium as a disk-shaped magnetic recording medium used in existing hard disks in which a substrate rotates at high-speed, pores 43 embedded with a magnetic substance must be aligned in concentric tracks 42 on a disk-shaped substrate 41, as shown in FIG. 5. In this method, starting points of anodization are formed by pressing against an Al substrate a stamper with protrusions patterned to align at a constant interval to form a plurality of concentric tracks having a single center, and the Al substrate is anodized to form pores arranged concentrically. However, on a disk-shaped substrate, the length of a track is different on the inner side and outer side of the disk. As a result, when pores are aligned in a single track at a constant interval, the number of pores varies among tracks. In other words, as shown in FIG. 6, although pores are regularly aligned at an equal interval (2R) within a track, pores between different tracks are not regularly aligned. Instead, there is a plurality of intervals, such as a wide interval among pores (2Rxe2x80x2) and a narrow interval among pores (2Rxe2x80x3), so that a regular alignment throughout the entire substrate cannot be realized. This can lead to a failure between the disk that rotates at high-speed and a head to write properly, or to an inability to retain information recorded on the disk.
The present invention relates to a patterned medium, in which the process to align pores on a substrate is significantly facilitated and in which the pores are regularly aligned throughout the entire substrate, in order to solve the problems described above. Furthermore, the present invention also relates to a patterned medium with high recording density in which, by using pores thus aligned, deviations in magnetic interaction between adjacent pores are reduced for all pores in order to retain magnetization of magnetic substance in all pores.
In accordance with one embodiment of the present invention, a cylindrical magnetic recording medium has pores that are regularly aligned. The present invention also relates to a cylindrical magnetic recording medium in which vertices formed by four randomly selected pores form a quadrilateral, a square, a rectangle or a rhombus.
In addition, the present invention relates to a recording and reproducing apparatus having a cylindrical magnetic recording medium.
The present invention also relates to a method for manufacturing a cylindrical substrate of a cylindrical magnetic recording medium with pores. In one aspect, the method comprises the steps of regularly forming pores in a surface of a three-dimensional substrate, and a process to embed a magnetic substance in the pores.
Moreover, the present invention relates to a method for manufacturing a cylindrical magnetic recording medium in which vertices formed by four randomly selected pores form a quadrilateral, a square, a rectangle or a rhombus.
The shape of the cylindrical magnetic recording medium may be any shape as long as it is similar to a cylinder.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.